Mercator has a good point! I you have not been cleaning the bore properly that can be an issue. With the Sniper Rifles we thoroughly cleaned the bores about every 6 months using Sweets 7.62 Bore Cleaner. (It has a High ammonia concentration) "DONT SNIFF IT!"
It will for sure clean the bore! But there is some caution if you ever use it. 1. Read the Directions 2. Be sure there is absolutely no other solvent cleaner or anything in the bore. (Because if there is it can etch the bore!) 3. Only let it be in the bore for the specified time period. 4. Be sure it is absolutely gone from the bore. But as Merc stated a fouled bore certainly could cause your issue. And normal cleaning solvents if the bore is fouled excessively normally will not accomplish the objective easily. The Gas Ring would not have any effect on the problem. But it is good you replaced them just the same!
Let us know what happens after the thorough cleaning. To add to this post you should not have any problems regarding the rounds and your barrel twist. The 1:9 will handle rounds 55-in the low 70 grains regarding weight and shoot them very well. Could be as Mercator Stated. Is this the first time this has happened? Sounds like it is likely fouling I swear some of the cheaper ammo manufacturers uses coal dust in their powders.
Which ammo was the side strikes shot with? But I still would have the throat checked on it. Just for piece of mind! And to protect the Bolt and Rifle. Keep Us Informed regarding your problem when you get back from the Range this week! We all learn from each others experiences.